Rename writers and content interfaces to generalize

BaseWriter is specified as:

/// An interface to write objects into some content format.
// ignore: one_member_abstracts
abstract class BaseWriter {
  /// A string representation of content already written to this (text) writer.
  ///
  /// Must return a valid string representation when this writer is writing to
  /// a text output. If an output does not support a string representation then
  /// returned representation is undefined.
  @override
  String toString();
}

There is also an issue #126 as an proposition to change it.

Actual writers, PropertyWriter, CoordinateWriter, GeometryWriter and FeatureWriter are mixins that implement BaseWriter.

However mixin writers define content stream method and the base writer output data access. So different purposes.

Some refactoring and remodeling is needed:

actual content stream writers are renamed => PropertyContent, CoordinateContent, GeometryContent and FeatureContent
these mixins no longer implement BaseWriter
BaseWriter is enhanced and refactored as #126 suggests
new class ContentWriter is defined

class ContentWriter<T> extends BaseWriter {
}

Now when actual content writer are needed they are referenced as ContentWriter<PropertyContent>, ContentWriter<CoordinateContent>, ContentWriter<GeometryContent> and ContentWriter<FeatureContent>.

This shall be breaking change but maybe worth it...

navibyte / geospatial

Rename writers and content interfaces to generalize #127